Question

Provide the definition of the second law of thermodynamics. How does the second law explain why heat travels from a substance at higher temperature to one at lower temperature?

Short answer

The second law of thermodynamics states that the total entropy of an isolated system never decreases. Heat flows from a higher to a lower temperature substance because this process increases the total entropy of the system, which aligns with the second law.

Step-by-step solution

Define the Second Law of Thermodynamics

The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. It can only increase or remain constant. Entropy is a measure of disorder or randomness, and this law implies that systems naturally progress towards a state of maximum entropy.

Explain Heat Flow in Terms of Entropy

According to the second law, when two substances at different temperatures come into contact, the heat will flow from the higher temperature substance to the lower temperature substance until thermal equilibrium is reached. This is because the flow of heat increases the overall entropy of the combined system. The higher temperature substance loses some entropy as it cools down, but the lower temperature substance gains more entropy than the loss from the hotter body, resulting in a net increase in entropy.

Key concepts

Entropy

Entropy, often symbolized as 'S', is a fundamental concept in thermodynamics that often perplexes students. In essence, entropy is the measure of disorder or randomness within a system. An everyday analogy might be a room that becomes more cluttered over time if no effort is made to tidy it. In thermodynamics, this translates to the idea that energy tends to disperse or spread out as much as possible.

According to the second law of thermodynamics, the total entropy within an isolated system will either increase or remain constant; it can never spontaneously decrease. Students can think of it as a universal principle that all natural processes are irreversible. When heat is transferred between two bodies, the dispersal of energy results in a change in the system's entropy. And considering the universe as the ultimate isolated system, its total entropy is always increasing.

This concept is critical when attempting to understand why, for instance, a hot cup of coffee left on a counter will eventually reach room temperature - heat (energy) flows out of the coffee (where it is concentrated) into the surrounding air, increasing the overall entropy of the room.

Thermal Equilibrium

Thermal equilibrium is a term used when two or more areas or objects at different temperatures come into contact and eventually reach a state where they are at the same temperature. This concept is crucial in understanding various physical phenomena and is a direct consequence of the second law of thermodynamics.

To visualize this, imagine two adjoining rooms, one significantly warmer than the other. If the door between them is opened, heat will flow from the warmer room to the cooler one until both rooms have the same temperature. The system then achieves thermal equilibrium. At this equilibrium point, there is no further net flow of heat between the areas, and entropy is maximized given the constraints of the system. Students should understand that equilibrium does not mean that heat flow stops entirely – it means that there is a balance, with heat flowing in both directions at an equal rate, resulting in no net change.

Heat Flow

Heat flow, also known as heat transfer, is the movement of thermal energy from one thing to another due to a temperature difference. It's an everyday experience, like feeling the warmth of the sun on your skin or the heat from a stovetop. In terms of thermodynamics, heat flow is a critical concept that gives more insight into the natural tendency of heat to spread out.

From the perspective of the second law of thermodynamics, when two systems at different temperatures are brought into contact, heat will flow from the hotter system to the cooler one until thermal equilibrium is reached. This heat flow increases the overall entropy of the combined system. For example, if a hot metal rod is placed in a cup of cold water, the metal rod cools down while the water heats up, and the entropy increases as the thermal energy is spread more evenly between the rod and the water.